The primary aims of this proposal are: 1) to determine whether hormone replacement therapy (HRT): testosterone in men, estrogen in women) enhances cognitive functioning and mood in physically healthy, cognitively intact adults over the age of 65 years; and 2) to better understand the neuroanatomical and neurophysiological substrates for the expected changes in cognition and mood by using magnetic resonance imaging (MRI) to measure brain structure and positron emission tomography (PET) and 18-F-Fluorodeoxyglucose (18-F-FDG) to measure regional cerebral glucose metabolism. The hypotheses are: 1) participants will perform better on memory tasks, including both standard neuropsychological tests and the activational memory task, while receiving hormone replacement therapy (HRT) than while receiving placebo; and 2) within-subject comparisons of the pattern of regional brain activity will differ during the hormonal treatment phase compared to the treatment phase with notable differences in brain regions known to subserve memory abilities, such as the medial temporal lobe.